Safety device for prevention of electrical shocks

ABSTRACT

A safety device can provide electrical shock protection for an electrical appliance. Voltage sensors can monitor voltage levels among a power conductor, a neutral conductor, and a ground conductor. When sensed voltages indicate that these conductors are properly wired to an electrical power utility, inline switches can close to allow power to pass through the safety device to the appliance. When one or more of the sensed voltages indicates that the power level is too low to operate the appliance, one or more switches of the safety device can open to block power from passing through to the appliance. When one or more of the sensed voltages indicates that an electrical problem posing a shock hazard exists in the electrical appliance, one or more switches of the safety device can open to interrupt power from transmitting to the appliance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 60/528,067, entitled “Safety Device to ProtectAgainst Shock from Improper Electrical Connections in ElectricalOutlets,” and filed Dec. 10, 2003. The subject matter of U.S.Provisional Patent Application No. 60/528,067 is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to the field of electrical devices, andmore specifically to safety devices that prevent electrical shock fromimproperly wired electrical outlets.

BACKGROUND

An electrical appliance or load receives electrical energy from oneterminal of an electrical outlet or source (the so-called high voltageor hot side), electrical current flows to the load through an electricalconductor (a wire), this current passes through the load and is thenreturned to another terminal of the electrical outlet through anotherwire called the neutral wire. The two wires that connect source and loadmay have a coating of rubber or some other electrical insulatingmaterial or they may be bare, in which case air, which is a goodinsulator, functions to inhibit electrical current flow outside of thewire. Since the human body can conduct the flow of electrical current,if a person comes into contact with one electrified object, such as theso-called hot wire in an electrical system, while also making contactwith a second object having a substantially different voltage, then anelectrical leakage current that is proportional to the voltagedifference will flow through the person and may cause injury or death.If the second object that the person comes in contact with iselectrically connected to the earth (“ground”) then this is called aground fault.

Electrical current is the flow of electrons. Electrons are neithercreated nor destroyed so any functioning electrical appliance willrequire both an entry path for electrons and an exit path for electronsin order for electrical current to flow. For most household appliancesthat operate from a plug, electrons will sometimes enter path one andexit path two and sometimes enter path two and exit path one. This isknown as alternating current or AC operation.

Although the two conductors coming out of an AC power source are oftendesignated as “hot” and “neutral”, in an AC system, the hot conductorwill cyclically have a more positive voltage than the neutral for halfthe time and will cyclically have a more negative voltage than neutralfor half the time, having a momentary value of zero (the so-called “zerocrossing”) each time the voltage passes from positive to negative orfrom negative to positive.

Any electrical appliance that plugs into a wall outlet can be the sourceof electrical shock, electrical burns, and possibly electrocution. Wornor damaged appliances or appliance cords may come in contact with aconducting surface that is accessible to the user, thus presenting ahazardous condition. For example, if the rubber insulation on a wirewithin an appliance is worn, then the exposed metal wire strands mighttouch the appliance housing. If the appliance housing were made out ofmetal or a similar electrical conducting material then a shock hazardcould exist.

In order to minimize the potential hazards intrinsic to electricalappliances, government and industry standards serve to regulate the wayin which appliances are built and used. One outcome is that many classesof electrical appliances are required to have a grounded connection overand above the standard two electrically conductive wires that supplyelectrical power to the appliance. A grounded electrical cord isrecognizable because it has a plug with three prongs. In the U.S., in a120 volt (sometime referred to nominally as 110 volts, 115 volts, or 120volts) single phase system, two of the prongs on a grounded plug areflat and the third, ground prong, is generally rounded.

To achieve electrical protection, the grounding line will connect toground at the plug where it is plugged into a grounded outlet. The otherend of the grounding line, within the cordset, will connect to theappliance housing or other exposed or potentially exposable metal parts.If an electrically hot (that is, having a voltage potential that issignificantly different from a ground potential) conductor comes intocontact with a grounded conductor, the grounded conductor will present alow resistance path to ground, causing the hot conductor to maintain avoltage at or near a ground potential. If the current drawn from anelectrically hot source, through ground, exceeds the “trip” current ofan in-line circuit breaker, the circuit breaker will open, removingpower from the outlet that the appliance is plugged into, therebyprotecting the electrical distribution system from over-current damageand protecting the user from electrical shocks.

If the ground connection on a grounded appliance is removed, the safetyof the appliance is compromised. This removal of ground condition canoccur through abuse or damage but more commonly occurs when theappliance is plugged into an outlet in which the ground has not beenconnected or is improperly wired. The removal of ground may also occurif the grounding prong is cut off of the plug or is otherwisedisconnected. An open ground condition often goes unnoticed by thecasual user. This is because most appliances will operate normallywithout a ground connection.

In the U.S., agencies such as the National Fire Protection Association(NFPA) and the Occupational Safety and Health Association (OSHA)maintain regulations governing the use of grounded appliances. For manysituations, the regulations require that a qualified technicianperiodically test the continuity of the ground connection betweenappliance and the grounding prong and between grounding prong and actualground.

One drawback to the periodic test approach is that a significant periodof time may elapse between inspections. In many situations theinspection schedule may not be adhered to or may be completely ignored.Also, malfunctions, particularly if intermittent, may go undetected.Often a complete inspection of the grounding impedance between actualground is not made. Accordingly, there are many situations where a usermay assume he is using a grounded appliance when he is not.

Another potentially hazardous operating condition occurs when the hotand the neutral conductors are switched at an outlet. The typicalappliance that is plugged into an outlet with these so-called“transposed” conductors will still operate. However, the design of manyappliances is such as to favor having one of the two power carryingconductors designated as the neutral conductor. For example, in adroplight, the socket for an incandescent light bulb will have theoutlet shell as neutral with the base connected to the hot conductor.Then if a user accidentally makes contact with the exposed outer shell,electrical shock is less likely to result. The neutral and hotconductors are distinguished by using a polarized plug wherein one ofthe two blades is designated as neutral and is physically wider. If adroplight is plugged into a polarized outlet but the hot and neutralconductors in that outlet are transposed, the safety associated withusing a polarized plug has been compromised. In such situations, it isimportant to alert the user.

In the prior art, U.S. Pat. No. 3,697,808 (Lee) discloses a system formonitoring chassis potential and ground continuity by injecting a radiofrequency (RF) signal into the neutral lead and monitoring the signalamount that is capacitively coupled to ground. A major limitation ofthis approach is that it is electronically complicated and is sensitiveto leakages through capacitances in the power cable and elsewhere.

U.S. Pat. No. 3,809,961 (Kershaw) discloses an electrical outletsentinel that detects an open ground condition in an electrical outletand opens a mechanical circuit breaker thereby removing power from theoutlet. A major objection to this design is that power from the outletis controlled by a circuit breaker whose relay is energized throughpower running continuously through ground. For any practical relay, thisrepresents a substantial continuous ground current and is likely todisrupt the correct operation of any ground fault circuit interruptersthat are upstream in the electrical distribution system.

U.S. Pat. No. 3,996,496 (Volk) discloses a ground integrity monitor thatrelies on the application of short electrical pulses between the neutraland ground terminals. If the resistance between neutral and ground isless than a threshold amount, this pulsing purportedly causes a photocoupler to be activated, providing power to the load. The system iselectronically complicated and this compromises the appeal of thedesign.

U.S. Pat. No. 4,598,331 (Legatti) discloses a ground fault interruptcircuit in which open ground conditions create actuation of theinterrupter. An open ground produces current flow through a supplementalsecondary winding that, in turn, induces a trip signal. Whenimplemented, this approach does not check for a good connection goingforward to the appliance, but only going back to the outlet. If, forexample, the grounding conductor on the appliance cordset is cut,thereby leaving the appliance ungrounded, this approach will not detectthat condition.

U.S. Pat. No. 4,649,454 (Winterton, et al.) discloses an open grounddetection circuit for appliances that utilizes a special plug with fourelectrical connections. Two of these are the standard hot and neutralprongs. A third prong, having the location and shape of a conventionalground prong, has two electrical conductors (the so-called groundingblade and sensing blade) that are separated by an insulating member.Four wires connect this special plug to a housing that is located withinan appliance. When the special plug is correctly seated in a three-holegrounded outlet, the two blades on the third prong are electricallyconnected together. If, however, the two conductors on the third prongare not electrically connected together, then an alarm signals that anopen ground condition exists. A major problem with this design is thatit only checks to see whether grounding blade and sensing blade areshorted together. This can occur even if the plug is connected to anungrounded outlet and the user would incorrectly assume that theappliance was grounded. Another problem with this design is that itrequires a special plug and four wires to connect between the applianceand this special plug.

U.S. Pat. No. 4,931,893 (Glennon) discloses a circuit that detects aloss of ground condition in an electrical system by using a capacitivecircuit that discharges through the ground. The disclosed embodimentsare electronically complex and can generate relatively high groundcurrents which would cause nuisance tripping in distribution systemshaving ground fault interrupters.

U.S. Pat. No. 5,844,795 and 5,943,198 and U.S. patent application Ser.No. 08/756,784, all to Hirsch et al., describe a solid state ground andarc fault detection and interruption technology that has two parts, onepart which resides in the load and one part which resides in the source.In applying this to an electrical appliance, the load is the applianceand the source is the plug. The basic theory in this technology is thata load conditioning module in the appliance injects a deadzone in thecurrent flow during each half wave AC cycle. A sensing circuit in theplug looks for the presence of that deadzone each half cycle. If thereis leakage around the load conditioning module (indicating a groundfault or arcing fault) this is indicative of a potential dangerouscondition and current flow is interrupted at the plug.

To address these representative deficiencies in the art, what is neededis an improved capability for controlling electrical power to anappliance in a manner that provides safety to people that may come incontact with the appliance.

SUMMARY OF THE INVENTION

The present invention supports controlling electrical power on a supplyline to an electrical appliance, such as a vending machine, tool,machine, equipment, motor, or other device or load that consumeselectrical energy, to protect against electrically shocking people thatmay contact the appliance.

In one aspect of the present invention, a safety device that helpsprevent electrical shock can comprise voltage sensing circuits thatmonitor voltage levels among or between a power conductor, a neutralconductor, and a ground conductor. When one or more of the sensedvoltages indicates that these conductors are properly wired to anelectrical power utility, inline switches can close to allow power topass through the safety device to the appliance. When one or more of thesensed voltages indicates that the power level is too low to supportproperly operating the appliance, one or more switches of the safetydevice can open to block power from passing through to the appliance.When one or more of the sensed voltages indicates that an electricalcondition in the electrical appliance poses a shock hazard or threat,one or more switches of the safety device can open to interrupt powerfrom transmitting to the appliance.

In another aspect of the present invention, the safety device canprovide shock protection for vending machines that operate in publicplaces. Upon occurrence of an event that poses a safety threat or issue,the safety device can isolate the vending machine from the electricalutility while keeping the connection to earth ground intact.Electrically isolating the vending machine can comprise opening theneutral conductor and the power conductor, which may also be referred toas a hot conductor or lead.

The discussion of providing protection against electrical shockpresented in this summary is for illustrative purposes only. Variousaspects of the present invention may be more clearly understood andappreciated from a review of the following detailed description of thedisclosed embodiments and by reference to the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of exemplary embodiments of the presentinvention. Moreover, in the drawings, reference numerals designatecorresponding parts throughout the several views.

FIG. 1 is an electrical schematic of an exemplary electrical safetydevice that provides protection against electrical shocks in accordancewith an exemplary embodiment of the present invention.

FIGS. 2A and 2B are electrical schematics of exemplary circuits for anelectrical safety device in accordance with an exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention supports providing electrical safety protectionfor an appliance by monitoring voltage levels on conductors that feedelectrical power to the appliance and actuating switches when thevoltage levels indicate the presence of an electrical safety issue.

This invention can be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thosehaving ordinary skill in the art. Furthermore, all “examples” givenherein are intended to be non-limiting, and among others supported byexemplary embodiments of the present invention.

The present invention can comprise a process or method related toproviding electrical safety protection. Certain steps in any exemplaryprocesses or methods described herein must naturally precede others forthe present invention to function as described. However, the presentinvention is not limited to the order of the steps described if suchorder or sequence does not alter the functionality of the presentinvention. That is, it is recognized that some steps may be performedbefore or after other steps or in parallel with other steps withoutdeparting from the scope and spirit of the present invention.

Turning now to FIGS. 1 and 2, circuitry for an exemplary electricalsafety device in accordance with an embodiment of the present inventionwill be described with reference to these figures. FIG. 1 illustrates aschematic of an exemplary safety device 100 that withholds electricpower from an appliance, such as a tool or electrical machine, if theelectric outlet 108 that it is plugged into is improperly wired. FIGS.2A and 2B illustrate schematics for exemplary circuits of the safetydevice 100.

The safety device 100 plugs into an electric outlet, typically wallmounted, and an appliance connects to the safety device 100. In oneexemplary embodiment of the present invention, the safety device 100provides power protection to a vending machine (not illustrated). Thesafety device 100 can be mounted on or in such a vending machine. Thatis, the vending machine and the safety device 100 can share a commonhousing. Alternatively, the safety device 100 can be external to avending machine.

The basic safety device 100 can be self-contained and designed to pluginto an electric outlet with the appliance receiving its electricalpower through the safety device 100. The safety device 100 can behard-wired to the appliance or to a receptacle that mates with a powercable. The safety device 100 can provide an essentially equivalent levelof protection against shock independent of its mounting configuration.

If the safety device 100 is configured in a plug-in format, the safetyprotection can be bypassed by simply unplugging the appliance from thesafety device 100 and plugging the appliance directly into the electricoutlet. Thus, when used on large electrical appliances, the safetydevice 100 is preferably hard-wired in the electric cable between theelectric outlet and the operating mechanism of the machine. For example,the device can be mounted inside the machine so that users can not betamper with it. A flashing light and remote audio alarm 120 can providean indication of the state of the safety device 100. That is, anindicator can provide a visible or audible signal that indicates whetherpower is properly or improperly wired to the appliance.

In a nonpermanent installation, the safety device 100 can also be usedas the outlet box on an extension cord. That is, the safety device 100can comprise an outlet that mates with a power cord of a tool or otherappliance. For example, hand-held power tools can plug into the safetydevice 100, thereby protecting users from shock. Again, if the electricoutlet that the safety box plugs into has been improperly wired, thetools will not receive any power. That is, the safety device 110prevents electrical power from transferring to the tool if theelectrical wiring that feeds the tool is miswired.

The safety device can comprise two or more indicator lights 128. One ofthe indicator lights 128 can emit red light as an indication that thepower feed lines 102, 104, 106 to the appliance are properly wired.Another indicator light 128 can emit green light to indicate that theappliance is properly wired or improperly configured.

If the voltages on the electric receptacle are the proper values whenthe appliance is plugged in, the safety device 100 will allow power totransmit to the appliance. If the voltages are incorrect, the safetydevice 100 blocks power from transmitting to the appliance. A user oroperator of an appliance connected to the safety device 100 will noticethe power interruption as a motivation to troubleshoot the problem andcorrect the wiring error. That is, the electrical wiring problem willneed to be corrected before a person or member of the public can operatethe appliance.

In addition to providing protection for connecting power to anappliance, the safety device 100 continuously monitors the voltages onthe power feed lines 102, 104, 106. That is, the safety device 100provides power protection after the appliance tool begins operating. Ifany of the measured voltages become incorrect, the safety device 100will remove power from the appliance. Thus, in one exemplary embodiment,the safety device 100 is not limited to checking power when theappliance is initially plugged in to an outlet. The safety device'scontinuous monitoring capabilities can protect against a wiring changeof the outlet that occurs subsequent to initially connecting electricalservice to the appliance. Thus, the safety device 100 can provideon-line, continuous monitoring of the electrical service to anappliance. This feature is useful for vending machines that operate inpublic places, for example.

In one exemplary embodiment, the safety device 100 comprises adifferential current detector. If the hot lead 106, which may also bereferred to as the power conductor 106, into the appliance comes intoelectrical contact with the metal frame of the appliance, a separatesafety switch opens up, removing the power. The voltage remains offuntil the differential switch is reset. This feature is alsocontinuously monitoring the circuit.

These features reduce the chance that a person will be shocked orinjured by electricity on an appliance equipped with the safety device100.

As discussed above, in one exemplary embodiment of the presentinvention, the safety device 100 provides electrical safety protectionfor a vending machine, such as a soft drink vending machine thatdispenses cold drinks in aluminum cans. Operation of the safety device100 in an exemplary vending machine application will now be discussed infurther detail below. Those skilled in the art should be able to applyelectrical safety protection in accordance with the present invention toa wide variety of tools, equipment, appliances, and machineryapplications following this discussion, the drawings, and the remainingdisclosure. For example, the safety device 100 can provide electricalprotection to a domestic refrigerator or freezer.

Referring to FIG. 1, the safety device 100 connects to a wall outlet orpower socket (not shown) via a polarized plug 108. The plug 108 can havethree prongs, one for the power line 106, one for the neutral line 104,and one for the ground line 102. These three lines or power conductors102, 104, 106 supply electrical power to the vending machine (notshown). The electrical power, which is typically 120 volts, single phasealternating current, passes through the safety device 100 on its path tothe vending machine.

The safety device 100 blocks, interrupts, or does not allow power topass through to the vending machine if the electric outlet is miswired.Further, the safety device can isolate the vending machine from theelectrical utility while maintaining a connection to earth ground. Thevending machine simply will not turn on or operate if the electricsupply source is not properly wired.

The safety device 100 comprises three voltage detection circuits 116,114, 130 one current differential circuit 108, and one alarm circuit120. The three voltage detection circuits 114, 116, 130 monitor, detect,or look for levels of voltage difference among the power or hotconductor 106, the neutral conductor 104, and the ground conductor 102.Each of these detection circuits 114, 116, 130 can monitor therespective voltage levels as compared to a threshold voltage, which canbe a minimal, trickle, or essentially zero voltage.

The first voltage detection circuit 116 monitors the voltage differencebetween the power conductor 106 and the neutral conductor 104. Thesecond voltage detection circuit 114 monitors the voltage differencebetween the power conductor and the ground conductor 102. The groundconductor 102 typically comprises an equipment ground and iselectrically connected to or comprises the frame of the vending machine.The third voltage detection circuit 130 comprises a low voltage detector132 that monitors the magnitude of the voltage on power and neutralconductors 106, 104.

If the proper voltages are present on the electrical service lines 102,104, 106 when the vending machine is first plugged into the outlet, thecontrol switches 118, 120 close and power is allowed to pass through tothe current differential circuit 108 and on to the operating mechanismsof the vending machine. In response to this power, the vending machine'slights will come on and its cooling compressor will start cooling.

If an improper voltage appears at any of the detection circuits 114,116, 130 when the vending machine is first plugged in to the outlet, thecontrol switches 118, 120 remain open and the safety device blocksvoltage from passing through to the vending machine. Thus, if the wiringconfiguration is backwards or otherwise wrong, the vending machine doesnot receive power and does not comes on. Having a vending machine thatis in a state of non-operation typically motivates a technician or otherservice personnel to inspect the electric outlet and address theproblem.

Once the vending machine is plugged in and is operating properly, thesafety circuit continuously monitors the supply voltages. If any supplyvoltage changes, the control switches 122, 128 open, thus removingelectrical power from the vending machine.

If the hot line 106 in the electric outlet becomes switched to thesafety ground contact, then one of the voltage detectors 1, 2 willtrigger an alarm circuit 120. A strobe light mounted on the exterior ofthe vending machine will start flashing. In addition, a wailing audiblealarm will sound off. This notifies people not to touch the vendingmachine.

In one exemplary embodiment of the present invention, the safety device100 outputs an audible warning message, for example “Do not touch thisvending machine,” via a digital recording coupled to a speech synthesischip or similar device.

While a flashing red light may provide an adequate warning of a safetyhazard to an adult, young children may not understand the significanceof the light. A young child might even be drawn to a siren or flashinglight. However, even a young child can understand a warning sentence,phrase, or word delivered in a verbal or spoken format. Such digitallysynthesized speech can have a harsh tone or other inflection thatchildren can recognize and appreciate.

In addition, the current differential detector 108 continuously monitorsthe current of both the power conductor 106 and the neutral conductor104, once the vending machine is functional. When the power currentequals to, or is within a threshold of, the neutral current, the vendingmachine operates in a normal manner. However, if the current through theone of these conductors 104, 106 becomes a certain amount more than thecurrent on other conductor 104, 106, the current differential monitor108 opens up the power conductor 106 to the vending machine. The lineremains open until the differential detector 108 is reset.

A properly wired electric outlet that provides 120 volt (nominal) singlephase alternating current (AC) will typically provide approximately 125volts AC between the power conductor 106 and the neutral conductor 104.The voltage detector 1 of the circuit 116 will detect this voltage andclose the switches 118 and 120 (switches S-1) upon determining that thevoltage indicates a proper wiring configuration. In a properly wiredelectric outlet, there will also be approximately 125 volts AC betweenthe power conductor 106 and the ground conductor 102. The voltagedetector 2 of the circuit 114 will detect this voltage and will closethe switch S-2 126.

The closure of the switch 116, the switch 120, and the switch S-2 126puts a voltage on relay K-4 116, which energizes and closes bothswitches S-4-1 118 and S-4-2 118. This action allows power to passthrough the safety device 100 to the vending machine and its internalcircuits. In operation, voltage detector 1 supplies 12 VDC to operaterelay K-4 and voltage detector 2 closes switch 126, thereby energizingrelay K-4 to pull in the switch 118 and the switch 120. That is, thetwelve volts from voltage detector 1 and switch 126 must be in the “on”or closed state at the same time to allow electrical power to pass tothe vending machine.

Thus, the safety device 100 prevents operation of the vending machine ifthe equipment ground wire is open or otherwise provides an inadequatelevel of grounding. In many circumstances, an improperly groundedvending machine (not shown) might continue to operate in a dangerousstate. Since many vending machines installations predate governmentregulations that mandate electrical grounding, the safety device 100 canhelp prevent electrical injury associated with these older machines.

The equipment ground conductor 102 is connected directly to the metalframe of the vending machine. Its purpose is to keep the frame of themachine at “earth ground” potential to prevent electrical shock. It doesthis by tying the frame all the way back to a ground buss on the mainbreaker panel with a conductive path that may be referred to as one“continuous wire.”

Thus, if the power conductor 106 accidentally comes into electricalcontact with the metal frame inside of the vending machine, the groundcircuit provides an electrical path back to the electrical systemground. This ground fault current should trigger the circuit breaker totrip, if and when that current reaches the breaker rating. Sucharrangement is specified by the National Electric Code (NEC).

The NEC specification are generally viewed as providing an adequatelevel of protection for the electrical conditions that may damageelectrical wires. However, an added level of protection, as provided bythe safety device 100, can help prevent electrical shock to people thatmay inadvertently come in electrical contact with a vending machinehaving improper wiring or a fault or other potentially hazardouscondition.

The safety device 100 can immediately detect an open equipment ground,issue a warning, and interrupt electrical service to a vending machine.When the equipment ground opens, the voltage detection circuit 114 doesnot close the switch S-2 114. When the switch S-2 114 opens, relay K-4116 drops out and power is removed from the vending machine. Powerremains disconnected until the electric outlet is rewired correctly.

Detection circuit 114 and detection circuit 116 provide a level ofprotection against shock that covers the majority of the wiring problemsthat are associated with electrical outlets. However, in one exemplaryembodiment, the safety device 100 comprises two additional circuits 130108 that provide an additional level of protection against shockhazards.

The low voltage detector 132 and its associated circuit 130 providelow-voltage protection. That is, the low voltage circuit 130 interruptspower to the vending machine if the voltage drops to an unacceptably lowlevel. Under such a low voltage condition, electric motors can drawexcessive current, heat up, and burn out. The voltage detector 132 opensthe switches 118, 120 of the control circuit, relay K-4 116, when thehot-to-neutral voltage falls below a predetermined value or threshold.The voltage detector circuit 116 can be an optional feature of thesafety device 100 for protecting electric equipment with motors and/orcompressors that are sensitive to low-voltage damage.

The second extra protective device is the differential current monitor108, which can be a built-in ground fault detector. If an un-insulatedsection of the power conductor 106 accidentally touches the metal frameof the machine, some current will flow through the equipment ground backto the ground buss. In this situation, the power conductor 106 willcarry essentially all of the incoming current and the return currentwill be divided between the neutral conductor 104 and the groundconductor 102. The differential current detector 108, which can bereferred to as a ground fault indicator (GFI), senses this difference incurrent and trips, thereby opening up the hot lead 106 and the neutrallead 104 to the vending machine. In comparison to opening only the hotlead 106, opening both the hot lead 106 and the neutral lead 104 offersan added level of safety protection. Thus, the safety device 100 canelectrically isolate a vending machine from the electrical utility,while keeping the earth ground intact, when a ground fault occurs. Inthe ground fault situation, switch 122 and switch 124 remain open untilreset and the ground fault is cleared. If the problem is not corrected,the differential current detector 108 will trip again as soon as it isreset. That is, the ground fault switches 122, 125 will continuetripping until the leakage path to ground is removed.

As will be appreciated by those skilled in the art, the level of safetyprotection that an individual GFI circuit provides can be inadequate incertain circumstances. However, including the GFI circuit in the safetydevice 100 can provide an enhanced level of shock protection. Withoutthe safety device, a GFI circuit can protect against a hot wire contactto the frame with the equipment ground open. However, such an individualGFI circuit can fail to provide adequate protection in a scenario inwhich the safety wire was open and the hot lead touches the metal frame.In this situation, the individual GFI will not trip open as there is noreturn path for the current. If a person touches the machine under theseconditions and completes the circuit to the earth ground, the GFI shouldtrip within one half of a cycle. In contrast, the safety device 100 canrecognize this situation and open switch 118 and switch 120, therebyremoving power from the vending machine entirely without needing aconductive path through the person.

The safety device 100 can further comprise a surge suppression device(not illustrated) that benefits the operation of the GFI 108. Some GFI'sare prone to tripping on conditions unrelated to local ground faults.When an electrical transient occurs on a power distribution network,power spikes can propagate to the safety device 100 and cause the GFI108 to trip. For example, a ground fault at a remote location on thepower grid can send a voltage spike on a hot line that can travel to thesafety device's GFI. The GFI 108 can misinterpret this spike as a groundfault. A surge suppressor, either integral to the safety device 100 orexternally connected to the power utility in the vicinity of the safetydevice 100, can suppress such voltage or current spikes or transients.The surge suppressor can operate by shunting the spike to ground, forexample. A silicon avalanche photodiode (SAD) is one exemplary type ofsurge suppressor. Metal oxide varistors are another exemplary form ofsurge suppressor that can be used with the safety device 100.

The safety device 100 has protection against faulty wiring in theelectric outlet, against an open equipment ground between the receptacleand the main circuit breaker panel, and against a high leakage path tothe frame within the vending machine. Further, the safety device 100 canbe installed without modifying the electrical receptacle or the vendingmachine. In one exemplary embodiment, vending machines can beretrofitted with safety devices 100, for example external to the vendingmachine frame or case. As discussed above, the safety device 100 can beused with a variety of household devices as a plug-in safety box.

Components of the safety device will now be discussed. In an exemplaryembodiment, the safety device 100 comprises a supply cable with anominal plug 108, a container box 110, and internal parts. When usedwith vending machines, an external flashing light and audible alarm 120can be wired into the circuit but be mounted directly on the vendingmachine. That is, the safety device 100 can comprise a tether,extension, or umbilical cord that leads to an alarm 120 that can besituated on a vending machine or another prominent location.

The power cable can have a standard 3-prong male plug. The larger flatprong connects to the neutral conductor 104. The smaller flat prongconnects to the hot conductor 106. The rounded longer prong connects tothe equipment ground conductor 102. The cable can comprise threestranded wires or leads, individually insulated, encased in aninsulating cover and capable of carrying the current load of themachine. The cable can be sized for compatibility with a circuitbreaker, for example a 20-amp breaker.

The cable enters one end of the safety box 110 and is secured to thebox's frame with a clamp (not shown). The individual leads connect tothe appropriate points on a control board (not shown) that holds thesafety device's various electrical components. The hot lead 106 connectsto terminal H1 134. The neutral lead connects to terminal N1 136. Theequipment ground lead connects to terminal G1 138. The control boardcircuits connect to these points 134, 136, 138.

The voltage detection circuit 116 between H1 134 and N1 136 resides onthe control board. The detection circuit 116 can comprise a simple120/12 volts alternating current (VAC) transformer, with a bridgerectifier to convert the low AC voltage to a direct current (DC)voltage. The DC voltage energizes K-1 relay 116 and closes switch S-1118, 120 when sensing voltages indicative of correct wiring. FIG. 2Aillustrates an exemplary circuit diagram or schematic of the circuitryassociated with the voltage detector 1.

The control board further carries the voltage detection circuit 114between H1 134 and G1 138. The detection circuit 114 can comprise asimple 120/12 VAC transformer, with a bridge rectifier to convert thelow AC voltage to DC voltage. The DC voltage energizes K-2 relay 114 andcloses switch S-2 when sensing voltages indicative of correct wiring.FIG. 2B illustrates an exemplary circuit diagram or schematic of thecircuitry associated with the voltage detector 2.

The voltage measuring circuit 130 determines the magnitude of the ACvoltage between H1 134 and N1 136. The detection circuit 130 measuresthe rectified DC voltage on the voltage detector 1. This DC voltage isproportional to and thus indicative of the AC voltage between H1 134 andN1 136. This voltage detection circuit 130 can comprise software orother digital control logic that avoids causing switch 118 and switch120 to open for momentary low voltage due to initial start up. Thiscapability can also provide automatic restarts when the voltage returnsto normal.

The GFI 108 also typically mounts on the control board. The lead fromthe switch S-4-1 118 connects to the hot contact on the supply side ofthe GFI 108. The lead from the switch S-4-2 120 connects to the neutralcontact on the supply side of the GFI 108. Switch S-5 122, 124 islocated in the GFI unit between the hot supply contact and the hot loadcontact. The GFI switches 122, 124 are normally closed contacts.

The hot lead from the load side of the GFI 108 connects to the H2terminal 140 on the control board. The neutral lead from the load sideof the GFI 108 connects to the N2 terminal 142 on the control board. Alead runs from the ground contact on the GFI 180 to the N1 terminal 136on the control board. A continuous lead runs from the G1 terminal 138 tothe G2 terminal 146 on the control board, without interruption.

An electric cable 112 carries power from the safety device to thevending machine. This cable 112 is brought into the safety device box110 and is clamped at the entry port. The leads in this cable 112 arehardwired onto the control board. The hot lead 106 for the vendingmachine cable connects to the H2 terminal 140 on the control board. Theneutral lead in this cable 112 connects to N2 terminal 142 on thecontrol board. The ground lead in this cable connects to G2 terminal 146on the control board.

As discussed above, the strobe and the alarm 120 are mounted on thevending machine where they can be seen and heard. The alarm leads, fromthe strobe and the wailing siren, connected to circuitry on the controlboard as illustrated in FIGS. 1 and 2A. Also as discussed above, the boxor housing 110 of the safety device 100 can be mounted to the frame ofthe vending machine.

The construction on the safety device can be straightforward. The powercable to the wall receptacle enters the box or housing 110 via a port orhole on one end and is secured with a clamp. A control board containingthe electronic components is mounted inside the box. The power cableleads 106, 104, 102 are separated and connected to terminals H1 134, N1136, and G1 138, as discussed above.

The control board is sized to hold the components selected, plus theterminal lugs for the leads. The components are mounted on the controlboard and wired according to the schematics. The control board ismounted in the safety box 110.

A power cable 112 from the vending machine equipment enters the box 110from the end of the safety box opposite the power supply end and issecured to the box 110 via a cable clamp or other restraining device.The leads of the vending machine cable 112 are separated and connectedto terminals H2 140, N2 142, and G2 146 on the control board.

The safety box 110 is sized to hold the control board and to allow forcable termination. The safety box 110 should be made from anon-conducting plastic material if it is to be used as part of anextension cord.

Although a system in accordance with the present invention can comprisea circuit that provides electrical safety protection to a vendingmachine or other appliance, those skilled in the art will appreciatethat the present invention is not limited to a specific application andthat the embodiments described herein are illustrative and notrestrictive. Furthermore, it should be understood that various otheralternatives to the embodiments of the invention described herein may beemployed in practicing the invention. The scope of the invention isintended to be limited only by the claims below.

1. A system that controls electrical power provided to an appliance viaa power conductor, a neutral conductor, and a ground conductor,comprising: a first detector that detects a first voltage between thepower conductor and the ground conductor; a second detector that detectsa second voltage between the power conductor and the neutral conductor;a first switch in series with the power conductor; a second switch inseries with the neutral conductor; and a coil that closes the firstswitch and the second switch if the first detector detects the firstvoltage and the second detector detects the second voltage.
 2. Thesystem according to claim 1, wherein the coil closes the first switchand the second switch if the first detector detects that the firstvoltage is above a first threshold and the second detector detects thatthe second voltage is above a second threshold.
 3. The system accordingto claim 2, wherein the first switch and the second switch are open ifthe first voltage is below the first threshold and the second voltage isbelow the second threshold.
 4. The system according to claim 1, whereinthe first switch and the second switch are normally open.
 5. The systemaccording to claim 1, wherein the system is operative to provideelectrical safety protection.
 6. The system according to claim 5,wherein the system blocks the electrical power from the appliance if theneutral conductor carries a voltage that is above a threshold.
 7. Thesystem according to claim 1, further comprising a ground faultinterrupter connected to the ground conductor, the power conductorbetween the first switch and the appliance, and the neutral conductorbetween the second switch and the appliance.
 8. The system according toclaim 7, wherein the ground fault interrupter opens a third switch, inseries with the power conductor, and a fourth switch, in series with theneutral conductor if an electrical fault occurs at the appliance.
 9. Thesystem according to claim 1, wherein the apparatus comprises a vendingmachine.
 10. The system according to claim 9, further comprising ahousing that encloses the first detector, the second detector, the firstswitch, the second switch, and the coil.
 11. The system according toclaim 10, wherein the housing is mounted to the vending machine.
 12. Thesystem according to claim 10, wherein the housing mounts to a walloutlet that supplies the electrical power to the vending machine via apower cable.
 13. The system according to claim 1, wherein the electricalpower comprises single phase power.
 14. The system according to claim 1,further comprising an indicator that provides a state indication for thefirst switch and the second switch.
 15. The system according to claim 1,wherein a relay comprises the first detector and the coil.
 16. Thesystem according to claim 15, wherein a second relay comprises thesecond detector.
 17. The system according to claim 16, wherein thesecond relay comprises a switch in series with the coil.
 18. The systemaccording to claim 1, further comprising a speech synthesis device thatoutputs a warning if the first detected voltage or the second detectedvoltage indicates a safety issue with the electrical power.
 19. A systemfor avoiding electrical shocks comprising: a detector that detectswiring configurations of an electrical utility to a vending machine; anda speech synthesis device, electrically coupled to the detector, thatissues a verbal warning in response to the detector detecting a wiringconfiguration that poses a safety threat.